(1) Field of the Invention
The present invention relates to a process for the preparation of perfluoro organic compounds.
(2) Description of the Related Art
In general, perfluoro organic compounds are chemically stable, have a high insulating strength, a high heat conductivity, a small surface tension and a high oxygen-dissolving power and is uncombustible, nontoxic and odorless. Because of these good characteristics, these compounds have been used as test bath media for electronic parts, heat media for the gas-phase soldering, insulating coolants for electronic appliances, oxygen carriers for artificial blood and operating fluids for heat pipes.
These perfluoro organic compounds are generally linear or cyclic perfluorocarbon compounds containing or not containing such atoms as nitrogen, oxygen and sulfur in the molecules.
In the instant specification and appended claims, perfluorocarbon compounds containing or not containing such atoms are inclusively called "perfluoro organic compounds".
Perfluoro organic compounds are prepared by substituting hydrogen atoms of an unfluorinated or partially fluorinated, saturated or unsaturated organic compound having a certain skeleton with fluorine atoms and/or adding fluorine atoms to this organic compound. The fluorinating means adopted in this preparation process is roughly divided into (i) a method using a fluorine gas, such as disclosed in Japanese Patent Application Laid-Open Specification No. 202122/85, J. Chem. SOC. chem. Commun., 1985(19) , 1350-2, J. Am. Chem. Soc., 1956, 78, 1679-82 or J. Am. Chem. Soc., 1985, 107(5), 1197-201, (ii) a method using a fluoride of a metal of a high atomic valency, such as disclosed in "Chemistry of Organic Fluorine Compounds", 2nd edition (1976), written by Milos Hudlicky and published by Ellis Horwood Limited, pages 77-82, and (iii) a method of electrochemical fluorination in HF, such as disclosed in the specification of U.S. Pat. No. 2,519,983 or the specification of European Patent No. 121,614.
In the fluorination method (i) using a fluorine gas, the reaction is generally violent. Accordingly, in many cases the liquid-phase reaction is carried out under such severe conditions that the temperature is controlled below 0.degree. C. and the reaction mixture is diluted, or the method (i) is used for partial fluorination for partial introduction of fluorine. If it is intended to effect complete substitution of hydrogen atoms in an organic compound having carbon-to-hydrogen bonds by using a fluorine gas, side reactions such as cleavage of the molecule and decomposition of functional groups are easily caused, and especially in case of amine compounds or the like, the yield is often drastically reduced.
In the fluorination method (ii) using a fluoride of a metal of a high atomic valency, such as cobalt trifluoride, a by-product is easily formed by decomposition, though not so conspicuous as in the method (i), and if complete fluorination is intended, drastic reduction of the yield is caused.
The electrochemical fluorination method (iii) is relatively simple. However, according to this method, the conversion to a perfluoro organic compound is fundamentally low, and it has been confirmed by us that when the formed perfluoro organic compound is separated in the form of a precipitate or gas from the electrolysis system, incompletely fluorinated compounds, for example, compounds having one or two hydrogen atoms in the molecule, are included in unexpectedly large amounts in the precipitate or gas. This phenomenon is especially conspicuous when the perfluoro organic compound is recovered in the form of precipitate.
In the case where a perfluoro compound is applied to the above-mentioned uses, an incompletely fluorinated compound as described above causes serious troubles even if the amount incorporated of the incompletely fluorinated compound is very small. For example, when the perfluoro organic compound is used as an oxygen carrier for artificial blood, because of the toxity of the incompletely fluorinated compound to the living body, it is necessary to remove the incompletely fluorinated compound completely. As means for removing the incompletely fluorinated compound, there are known some method in which the fluorination product is treated for example with an alkali metal or alkaline earth metal. However, if these methods are adopted, the loss of the perfluoro organic compound cannot be avoided, and a problem of reduction of the yield arises.